Recently, in industries including automobiles, home appliances, and construction materials, surface-treated steel sheets obtained by treating basal sheets to give antirust properties, in particular, galvanized steel sheets and alloyed galvanized steel sheets that can be manufactured at low cost and are excellent in antirust properties, have been used. Also, from the viewpoint of the improvement of the fuel efficiency and collision safety of automobiles, the demand has been growing for stronger materials for car bodies, thinner steel sheets based on such materials, and resultant lighter and stronger car bodies. To satisfy this demand, the use of high-strength steel sheets in automobiles has been promoted.
In general, galvanized steel sheets are manufactured by recrystallization annealing and subsequent galvanization of basal steel sheets, which are thin steel sheets obtained by hot-rolling or cold-rolling of steel slabs, in a continuous galvanizing line (hereinafter, referred to as CGL) equipped with an annealing furnace. As for alloyed galvanized steel sheets, the manufacturing process further includes alloying after galvanization.
Here, heating furnaces that can be used as the annealing furnace of CGLs include DFF-type ones (direct firing furnaces), NOF-type ones (non-oxidizing furnaces), all-radiant-tube-type ones, and so forth. Recently, however, CGLs equipped with an all-radiant-tube-type heating furnace have been favored for several reasons. For example, they are easy to operate, unlikely to cause pick-ups, and advantageous in other ways, thereby making it possible to manufacture high-quality plated steel sheets at low cost. Unfortunately, unlike DFF-type ones (direct firing furnaces) and NOF-type ones (non-oxidizing furnaces), all-radiant-tube-type heating furnaces do not support oxidizing before annealing and thus are disadvantageous to steel sheets containing Si, Mn, and/or other oxidizable elements in terms of the completeness of plating.
For exemplary methods for manufacturing hot-dipped steel sheets whose basal sheet is a high-strength steel sheet containing large amounts of Si and Mn, PTL 1 and PTL 2 have disclosed methods in which the heating temperature in a reduction furnace is specified for a higher dew point in accordance with its relation to the partial pressure of vapor so that the internal oxidization of the superficial portions of the basal sheet can take place. However, the resultant steel sheets, with oxides contained therein, are likely to crack during processing, inferior in the capability of retaining plating, and sometimes inferior in anticorrosive properties.
Also, PTL 3 has disclosed a method in which the concentration of CO2 is specified besides those of gaseous oxidants, namely, H2O and O2, so that the internal oxidization of the superficial portions of the basal sheet can take place just before plating and that the external oxidization can be prevented for an improved appearance of the resultant plating. However, as with those produced in accordance with PTLs 1 and 2, steel sheets produced in accordance with PTL 3 are likely to crack during processing because of oxides contained therein, inferior in the capability of retaining plating, and sometimes inferior in anticorrosive properties. Worse yet, CO2 contaminates the furnace, induces the surfaces of steel sheets to be carburized, and leads to other unfavorable events, thereby causing problems such as varying mechanical properties of the resultant steel sheets.
Furthermore, recently, the use of high-strength galvanized steel sheets and high-strength alloyed galvanized steel sheets in components that are subject to heavy-handed processing has been promoted, and thus the capability of retaining plating during heavy-handed processing has become increasingly important. More specifically, plated steel sheets should be able to well retain plating even if they are bent at an angle greater than 90° and subsequently further bent to make a more acute angle or subjected to impact.
To satisfy these performance requirements, not only should a large amount of Si be added to steel to provide steel sheets with an intended structure but also the superficial portions of the basal steel sheet, namely, the portions located in direct contact with the plating layers, which may provide starting points of cracks and other kinds of defects during heavy-handed processing, should have a well-controlled structure. Unfortunately, known methods have all failed to provide an easy way to control the structure of the superficial portions of the basal steel sheet; with a CGL the annealing furnace of which is an all-radiant-tube-type heating furnace, it has been impossible to process Si-containing high-strength basal steel sheets into galvanized steel sheets that can retain plating even during heavy-handed processing.